Crystal structures and magnetic properties of epitaxial Co-W perpendicular films

According to previous reports, Co-W alloy films with the hexagonal close-packed (hcp) exhibit high magnetic anisotropy and magnetically induced phase separation due to the miscibility gap formed along the magnetic phase transition, both of which are favorable for high density magnetic recording media. In the present study, the crystal structure, magnetic anisotropy energy (MAE), and their correlation are systematically investigated for Co _100-xW_x films (x=5, 10, 15) epitaxially grown on MgO (111) single crystals covered with Ru buffer layers by DC magnetron sputtering. The MAE strongly depends on the films thickness as well as the W content x and exceeds that of pure hcp-Co. Comparing with the elaborately analyzed crystal structures, such as lattice parameters (a, c), crystal axis ratios c/a, crystal axis dispersion, volume ratio of the fcc phase to the hcp phase, and deformation/growth stacking fault densities, the MAE is found to be closely correlated with the crystal axis ratio c/a, being qualitatively consistent with the classic single-ion anisotropy model and the recent first principles calculations.